The present invention relates to isocyanate modified epoxy resins (epoxy-terminated polyoxazolidones) having from about 5 to about 30 weight percent of the isocyanate content wherein from about 0 to about 50 percent of the isocyanate groups are converted to isocyanurate rings and from about 50 to about 100 percent of the isocyanate groups are converted to oxazolidone (commonly referred to in the past as "2-oxazolidinone") rings and a process for the preparation thereof. The present invention also relates to epoxy resin varnish compositions containing these isocyanate modified epoxy resins and electrical laminates prepared therefrom.
The synthesis and properties of isocyanate modified epoxy compounds which are copolymers of polyisocyanates and polyepoxides have attracted a great deal of interest in recent years. Various isocyanate modified epoxy compounds are described as promising engineering materials for the manufacture of a wide variety of useful products including rigid foams, thermoplastics, coatings, adhesives and elastomers.
During the reaction of a polyepoxide with a polyisocyanate there is competition between formation of 2-oxazolidone rings and isocyanurate rings. The crosslinking density and branching of the formed polymer compound increases with increased content of isocyanurate rings and thereby the glass transition temperature (Tg) of the polymer is raised. Many studies have shown that many factors such as reaction temperature, nature of the catalyst, relative amounts of the reactants and solvents employed, influence the reaction and formation of 2-oxazolidone rings and isocyanurate rings. See, for example, Kinjo et al, Polymer Journal Vol. 14, No. 6, 505-507 (1982); Sandler, S. R., Journal of Polymer Science, A 1-5, 1481 (1967); Sayigh and Ulrich, J. Chem. Soc., 1961, 3148-51; Uribe and Hodd, Polym. Sci. Technical, 31, 251-73 (1985); Kordomenos et al, Macromolecules, 14, 1434-37 (1981); Kitayana et al, Rubber Chemistry and Technology, Vol. 53, 1-6 (1980); Herweh et al, Tetrahedron Letters, No. 12, 809-812 ( 1971); Dileone, J. Polymer Sci., A 1-8, 609-615 (1970); Herweh et al, J. Org. Chem., Vol. 33, 1409-23 (1968) and Sandler, J. Polym. Sci., A1-5, 1481 (1967). For convenience most of this fundamental studies were conducted with model compounds in dilute solutions particularly in dimethylformamide (DMF).
In general it has been found that epoxides promote formation of isocyanurate rings at low temperature. It has been shown that isocyanurate rings are formed from the isocyanate groups by the catalytic effects of nucleophilic compounds at temperatures below 100.degree. C. and are converted into oxazolidone rings at temperature above 110.degree. C. in the presence of DMF solvent. It has been disclosed that the usual reaction temperature to form oxazolidone rings is about 150.degree. C. to 160.degree. C. (boiling temperature of DMF solvent).
There are three end products which can be obtained in the condensation reaction of polyisocyanates with polyfunctional epoxides. The reactants' stoichiometry (ratio of epoxide groups to isocyanate groups) determines the various end products, their remaining end groups and their possible uses. These condensation products are conventionally referred to as polyoxazolidones.
Isocyanate-terminated polyoxazolidones, are prepared using a stoichiometric excess of the polyisocyanate (isocyanate/epoxide ratio greater than 1) in the presence of a suitable catalyst. The following catalysts have been reported to promote the formation of isocyanate-terminated polyoxazolidones: quaternary ammonium salts, diethylzinc, zinc chloride/trimethylamine complex and organoantimony iodide. See, for example, Sander, J. Polym. Sci., A 1-5, 1481 (1967); Kordomenos et al, Journal of Coating Technology, 55, No. 700, pp. 49 and 59 (1983); U.S. Pat. No. 4,022,721 to Mitsubishi; and PCT Patent Application WO/US 86/06734 to Dow. Isocyanate-terminated polyoxazolidones are reported to be suitable for use as resins, elastomers, paints and adhesives. The isocyanate-terminated polyoxazolidones can be further cross-linked via the catalytic trimerization of the terminal isocyanate groups to isocyanurate rings.
Linear high molecular weight polyoxazolidones are prepared using stoichiometric amounts (isocyanate/epoxide ratio=1) of a diepoxy compound and diisocyanate compound.
Another class of resins (often referred to as "ISOX" resin) contain both isocyanurate and oxazolidone rings but the isocyanurate rings are predominant. The mechanical properties of ISOX resins can be varied from hard to flexible by changing equivalent ratio of isocyanate and epoxide groups. See, for example, Kinjo et al, Polymer Journal, Vol. 14, No. 6, 505-507 (1982). This reference describes preparation of a series of thermosetting ISOX resins in which diphenylmethane-4,4'-diisocyanate (MDI) was reacted with different diepoxides in the presence of ethyl-methyl imidazole as a catalyst and as a curing agent. ISOX resin is low viscosity liquid at room temperature before curing. Compared with other conventional heat resistant solventless varnishes, ISOX resin are reported to have greater mechanical strength at elevated temperatures, and goods heat, electrical, chemical and moisture resistance properties.
U.S. Pat. No. 4,070,416 (Hitachi Ltd.) describes a process for producing thermosetting resins by mixing one equivalent or more of polyfunctional isocyanate per one equivalent of a polyfunctional epoxide in the presence of a tertiary amine, morpholine derivatives or imidazole as catalysts. The catalyst is used within a range of 0.1 to 2 weight percent, based on the combined weight of the reactants. The reaction temperature of 130.degree. C. or lower is said to result in the formation of mainly isocyanurate rings, whereas it is assumed that oxazolidone rings should be mainly formed at temperature above 130.degree. C. The produced resins are said to exhibit excellent electrical and mechanical properties and high thermal stability. The produced resins are said to have various applications as heat resistance insulation varnishes, casting resins, impregnation resins, molding resins for electrical parts, adhesives, resins for laminating boards, resins for printed circuits etc.
Epoxy-terminated polyoxazolidones are prepared by reacting an epoxy resin with a polyisocyanate compound using stoichiometric excess of epoxy resin (isocyanate/epoxide ratio lower than 1). The following catalysts have been described in the literature to promote formation of epoxy-terminated polyoxazolidones: quaternary ammonium salts, HEXA (hexamethylenetetramine), diazabicyclooctane (sold by Air Product under the trademark DABCO), diethylzinc and organoantimony iodide. See, for example Sander et al, J. Appl. Polymer Sci., 9, 1984-1996 (1965); Uribe et al, Polym. Sci. Technical, 31, 251-73 (1985); Kordomenos et al, Journal of Coating Technology, 55, No. 700, pp. 49 and 59 (1983); and PCT Patent Application WO/US 8606734. The epoxy-terminated polyoxazolidones can be cured with conventional epoxy hardeners. The cured epoxy resin exhibit very good thermal properties and solvent stability and have generally good impact properties. The epoxy-terminated polyoxalidones are reported to be useful for the preparation of coatings resins and adhesives.
European Patent Application No. 0113575A discloses powder coating compositions comprising epoxy-terminated polyoxazolidone resins prepared by reacting a diepoxide and a diisocyanate in amounts which provide a ratio of epoxide equivalents to isocyanate equivalents of from about 1.1:1 to about 10:1 and curing agents. The polyoxazolidone resins are said to have comparatively high glass transition temperatures and provide coatings of improved resistance to cathodic disbandment. The coating composition are applied by fluidized bed sintering or electrostatic spray methods.
Self thermosetting compositions of polyisocyanates and polyepoxides are described in U.S. Pat. Nos. 4,564,651 and 4,631,306 (both to Markert et al) which discloses a method for the preparation of reaction resin molded materials and molded materials for insulating components, respectively containing oxazolidone and isocyanurate rings by mixing a polyepoxide and a polyisocyanate to from a resin mixture having a viscosity up to 7000 mPa.s at 25.degree. C. and the mole ratio of epoxy to isocyanate groups of from about 1:1 to about 5:1; reacting the resin mixture in the presence of an imidazole or tertiary amine catalyst at elevated gelling temperature of from 80.degree. C. to 130.degree. C. to form a cross-linked polymer; and heating the cross-linked polymer to from about 130.degree. C. to about 200.degree. C. to cause posthardening and produce a molded material. The molded materials are reported to exhibit improved mechanical properties.
U.S. Pat. No. 3,334,110 (Schramm) discloses a method for preparing epoxy-terminated polyoxazolidones by reacting a polyisocyanate with a polyepoxide in the presence of a catalyst mixture comprising an alcohol and tertiary amine or a quaternary ammonium salt. The epoxy-terminated polyoxazolidones can be cured with epoxy curing catalysts or reacted with epoxy hardeners to give a variety or products useful in the fields of coatings, laminating, bonding, molding, foams etc.
U.S. Pat. No. 4,066,628 (Ashida et al) discloses a process for preparing polyoxazolidones by reacting an organic isocyanate with an epoxide in the presence of dialkyl zinc, zinc carboxylate, organozinc chelate compound or trialkyl aluminum as the catalyst. Polyoxazolidones prepared by this process are useful starting materials for the manufacture of a wide variety of products including foams, coatings, adhesives, elastomers and the like.
Although numerous processes for the preparation of polyoxazolidones are described in the literature and various catalysts are used in these processes, only polyoxazolidones having fixed product specifications are produced. None of the known processes allows for the preparation of epoxy-terminated polyoxazolidones in controlled manner by manipulating various process parameters. None of the known processes provides for producing epoxy-terminated polyoxazolidones which will have particularly desired physical and mechanical properties by controlling the content of the oxazolidone and isocyanurate rings incorporated in the epoxy-terminated polyoxazolidone in order to accommodate desired end use of the polyoxazolidone. Furthermore, none of the known art discloses nor suggests that imidazole compounds would be useful as catalysts in a process for the preparation of epoxy-terminated polyoxazolidones. Moreover, there is no disclosure nor suggestion in the known art that epoxy-terminated polyoxazolidones would be useful materials for the preparation of electrical laminates and in particular, for the preparation of electrical circuit boards.
Heretofore electrical laminates have been conventionally prepared from bisphenol A based epoxy resins advanced with either bisphenol A or tetrabromobisphenol A and cured with a biguanide such as dicyanodiamide. However, for many applications, such as multilayered printed circuit boards, the glass transition temperature (Tg) of the cured epoxy resins is undesirably low, for example, less than 135.degree. C. and the chemical resistance of the cured epoxy resins is also undesirably low, for example, N-methylpyrrolidone pick-up is greater than 0.5 weight percent. Although in many applications these physical properties are acceptable, for many applications it is desirable to have cured epoxy resins which have glass transition temperature above 135.degree. C. and chemical resistance such as N-methylpyrrolidone pick-up of less than 0.1 weight percent. There are available specialty epoxy resins and other resinous products which meet the desired higher standards but their use is limited due to the economics. In particular, the cost associated with these resins precludes them from being used in the most of the applications associated with electrical circuit boards.
There is a need in industry for a process which will allow for the preparation or epoxy-terminated polyoxazolidones in a targeted manner by controlling various process parameters to obtain materials having physical and mechanical properties targeted to specific, desired end uses of such materials.
There is also a great need for engineering materials which will have the desired physical and mechanical properties (particularly, sufficiently high Tg's and chemical resistance properties) and still be inexpensive enough to be used in the most of the electrical laminate applications, particularly in electrical circuit board applications.
The present invention provides such engineering materials which materials have physical and mechanical properties targeted to specific end uses. Moreover, the present invention provides a process for the preparation of these engineering materials which process can be controlled in a targeted manner by manipulating various process parameters.